Liquid crystal devices (LCDs) normally comprise a thin cell containing a liquid crystal material, the upper and lower inside faces of the cell carrying (usually transparent) orientation layers. These innermost layers impart a preferred orientation to liquid crystal molecules in their vicinity by defining the actual arrangement of the liquid crystal director close to the boundary. This preferred orientation tends to persist even away from the orientation layers due to the strong interaction of the liquid crystal molecules.
The electro-optical effect in LCDs is substantially determined by the angle of twist through which the liquid crystal molecules pass from one side of the substrate to the other. Especially the contrast, brightness, viewing angle dependency and speed of the display, as well as the voltage required to actuate the liquid crystal display, can be adjusted to an optimum by means of the angle of twist. The liquid crystal properties required to obtain the electro-optical effect, such as the optical or the dielectric anisotropy, are determined by the angle of twist.
In order to establish a desired angle of twist, a preferred direction must be imposed on both substrate sides in contact with the liquid crystal. For that purpose, it is customary to apply to both substrate sides a thin polymer layer which is then rubbed in one direction, for example, with a cloth. Liquid crystals in contact with the orientation layer become oriented according to that preferred direction. The liquid crystal molecules must be sufficiently strongly anchored to the orientation layer for the molecules on the substrate surface to remain oriented in the desired direction, although the directions of orientation on the two sides of the substrate are generally different and, as a result, restoring forces occur. In that manner, it is possible to produce left- or right-rotating liquid crystal layers having an angle of twist of up to about 89°. At angles of 90° and above between the directions of orientation of the two substrates, the problem arises that the twist can occur either to the left or to the right, which, especially in the commercially widely available 90°-twisted liquid crystal displays, can result in areas being produced in which the liquid crystal rotates in the wrong direction (reverse twist), which leads to light scatter and a spotty appearance of the display.
Uniaxially rubbed polymer orientation layers, such as polyimide are conventionally used to orient liquid crystal molecules in liquid crystal displays. Although polyimides are very suitable as orientation layers by virtue of their good orientation properties there are a number of serious disadvantages that have less to do with the material itself than with the rubbing technique used to obtain the orientation. For example, in high-purity production environments, besides the dust produced during the rubbing process, electrostatic charges are generated on the surface of the substrate which attracts additional dust as well as interferes with the functioning of thin-film transistors (TFT) integrated under each pixel in LCDs.
The rubbing method is also subject to limitations because the increase in the miniaturization of LCDs, especially for use in projectors, and the growth in the number of pixels for high-resolution displays are resulting in ever smaller electrode structures, the dimensions of which are, in some cases, distinctly smaller than the diameter of the brush hairs used for the rubbing. Because of the topology of the substrate surfaces in TFT-LCDs, which is determined by the structure of the thin-film transistors, there are, for example, shadow areas that cannot be rubbed at all by the coarse fibers.
The problems associated with rubbed orientation layers can be solved using the photoalignment technique reported in U.S. Pat. No. 5,539,074 and International Publication No. WO 9949360 by Schadt et al., U.S. Pat. Application No. 2004/0219307 A1 by Shin et al., U.S. Pat. Application No. 2004/0213924 A1 by Nam et al., U.S. Pat. Application No. 2003/0021913 A1 by O'Neill et al., and U.S. Pat. Nos. 5,389,698 and 5,838,407 by Chigrinov et al, all incorporated herein by reference. In this method, anisotropic surfaces of the aligning layers are created using polarized light or a combination of polarized and non-polarized light irradiation. Cinnamate- and coumarin-containing polymers are usually used for this photoalignment technique because of their high photo- and thermal-stability of the induced alignment. Stable anisotropy in such materials is induced through the photo-dimerization (crosslinking) of photosensitive units. This method teaches the use of polymers, preferably of high molecular weight and soluble in organic solvents that are applied to a substrate and then photo-irradiated using ultraviolet light of wavelengths between 300-350 nm.
Another method used in the art for photoaligning is reported in U.S. Pat. No. 6,001,277 by Ichimura et al., incorporated herein by reference, uses a resin that contains a photoisomerizable and dichroic structural unit. Ichimura '277, however, does not disclose a resin with a photo-crosslinkable group, or applying an unreacted material to the substrate and then reacting and aligning by photo-irradiation. Polarized irradiation of anthracenyl containing polymers has been used for photoalignment. Thus, U.S. Pat. No. 5,928,561 by Bryan-Brown et al., incorporated herein by reference, uses a resin that contains a anthracenyl group which upon polarized irradiation at 325 nm results in photoalignment of liquid crystals.